1. Field of the Invention
The invention relates to a method and corresponding apparatus for electrolytic pickling a metallic strip, such as a stainless steel, titanium, aluminum or nickel strip, in which electric current is conducted through the strip indirectly without any electrically conductive contact between the strip and the electrodes which induce the current.
2. Description of the Related Art
In conventional systems for pickling cold-rolled stainless steel strips, a so-called "neutral electrolyte" process is used, in which a voltage is induced indirectly in the strip with no points of contact existing between any conductor rolls and the strip. In the conventional system, the anodes and cathodes used to induce the voltage are mounted horizontally in the pickling cell, and completely covered with electrolyte liquid.
For example, as shown in FIG. 1, in a conventional tank 1, a metal strip 2 is guided through the electrolyte 3 (e.g., Na.sub.2 SO.sub.4) between the cathodes 4 and anodes 5. The spacing between the electrodes and the strip is usually between approximately 70 and 150 mm, with the strip usually showing a certain amount of sag which can be reduced by inserting supporting rolls, for example, in the center of the plant. A pump 7 feeds the electrolyte 3 through a pipe 6 into the pickling tank 1. The electrolyte 3 is drained off through a pipe 8 and fed into a storage tank 9, for example, from where the electrolyte 3 is then recirculated.
A conventional process for electrolytic galvanizing a strip is described in published Austrian patent application no. AT-PS 373 922. In this process, the electrodes are mounted vertically, and electrolytic liquid is fed into the gap between the anode electrodes and the strip. However, a voltage is induced directly in the strip, with the conductor rollers acting as the cathodes.
A method for pickling a stainless steel strip at higher current densities is described in U.S. Pat. No. 4,363,709 to Zaremski. Although the patent describes a method which provides current densities of 40-60 A/dm.sup.2, the patent does not give any details on the equipment with which such current densities could be realized in a large-scale plant within a reasonable voltage range (less than 40 volts).
Accordingly, a continuing need exists for an effective method for pickling or galvanizing a metal strip by indirectly inducing a voltage in the metal strip.